I think a jumper between the two liquid out ports would work, filling the other keg from the bottom up. I'd run at low pressure so you don't lose too much CO2, or wait until it's in the second keg to carbonate.

Keep the pressure on both keps approximately the same before you start. Hook up your jumper from out post to out post then open PRV on receiving keg and away it flows. Apply pressure (5-10psi) to donor keg to keep the beer flowing.